The PD-L1 ligand binds to the PD-1 receptor, encoded by the PDCD1 gene (also known as CD279), which is present on T lymphocytes, B lymphocytes, natural killer (NK) cells, macrophages, and some subtypes of activated dendritic cells; as a result, PD-L1 participates in the PD-1/PD-L1 signaling pathway, which plays an important immunoregulatory role by suppressing the activation of immune cells in normal physiological contexts and in diseases, such as cancer [201,202,203,204,205]. The gene discussed is PDCD1; the disease is cancer.